The object of this invention is to provide a heat resistant austenitic stainless steel with high strength at elevated temperatures, good steam oxidation resistance, good fire side corrosion resistazce and a sufficient structural stability.
This invention also relates to a structural member of a boiler made of such heat resistant austenitic stainless steel with high strength at elevated temperatures, good steam oxidation resistance, good fire side corrosion resistance, and sufficient structural stability. Such a structural member could for instance be in the shape of an extruded seamless tube.
Austenitic stainless steels have been widely used for example as superheater and reheater tubes in power plants. In order to increase efficiency and meet environmental requirements, power plants will be required to operate at higher temperatures and under higher pressures. As a result, the material used in this type of installations requires improved properties regarding creep strength and corrosion resistance, since the conventional austenitic stainless steels such as AISI 347, AMSI 316 and AISI 310 will not be able to meet these higher demands. Various development efforts have been and are being performed in order to meet these tendencies towards more severe operation conditions in the power plant.
In general the precipitation of carbonides and solid solution hardering through addition of molybdenum and tungsten is effective for improving the strength of austenitic stainless steels at elevated temperatures. In addition there have also been improvements of the strength by adding considerable amount of copper to austenitic stainless steel. Chromium is the essential element used for improving the oxidation and corrosion resistance in high temperature alloys. Furthermore, the nickel content required for ensuring a structurally stable austenitic structure has been reduced in some previously developed alloys, due to substituting with nitrogen.
Generally it is difficult to obtain a corrosion resistant material with a high creep rupture strength that also has an acceptable structural stability, even when nitrogen is added as substitute for some of the expensive nickel. A rather high amount of nickel is needed in this material, with high levels of ferrite forming elements such as chromium, tungsten and niobium in order to suppress the formation of brittle phases such as the sigma phase after long term exposure. Chromium is added for big corrosion resistance and tungsten and niobium for high creep rupture strength. Other sigma phase promoting elements such as silicon and molybdenum have been held low while some elements, other than nickel have been added for the purpose of improving the structural stability.
The present invention provides an alloy with high creep rupture strength at elevated temperatures for long periods of time, a good steam oxidation resistance and fire side corrosion resistance and a sufficient structural stability.
An austenitic stainless steel according to the present invention comprises (by weight) 0.04 to 0.10% carbon (C), not more than 0.4% silicon (Si), not more than 0.6% manganese (Mn), 20 to 27% chromium (Cr), 22.5 to 32% nickel (Ni), not more than 0.5% molybdenum (Mo), 0.20 to 0.60% niobium b), 0.4 to 4.0% tungsten (W), 0.10 to 0.30% nitrogen (N), 0.002 to 0.008% boron (B), less than 0.05% aluminium (Al), at least one of the elements magnesium (g) and calcium (Ca) in amounts less than 0.010% Mg and less than 0.010% Ca, the balance being iron and inevitable impurities. Optionally, 2.0-3.5% copper (Cu) and/or 0.5% to 3% cobalt (Co) and/or 0.02-0.1% titanium (Ti) could be included.
In one embodiment of the present invention, the austenitic stainless steel has a composition that consists essentially of the above-listed constituent elements.
In further embodiment of the present invention, the austenitic stainless steel has a composition that consists of the above-listed constituent elements.